The recognition that pain is experienced by preterm infants and that adequate pain management can improve both short and long term outcomes has led to the more common use of pain controlling substances in this vulnerable population. Recently, an increasing appreciation for opioid-associated adverse events has prompted an increase in the use of acetaminophen (APAP) for treating pain in preterm neonates. However, there is far less information available concerning developmental aspects of metabolism and potential toxicity of APAP in preterm neonates as compared to full term infants, children and adults. In addition, very recent data have shown that elevations of APAP-CYS (a specific biomarker for APAP toxicity) in serum occur not only in association with severe APAP overdose but even, after therapeutic exposure of APAP in adults. In order to enhance the safety and efficacy of APAP use in preterm neonates, there is a need for an improved understanding of the developmental and pharmacogenetic determinants of age-associated differences in the metabolism and potential toxicity of APAP in this vulnerable population. The measurement of APAP-CYS combined with the use of metabolomic profiling and urine and serum metabolomics to identify toxicity-associated drug metabolite profiles or new biomarkers will provide new information pertinent to assessing the safety of APAP in preterm neonates. The clinical investigations proposed in this application have the following aims: 1. To evaluate the relationship of developmental stage (defined by both gestational, and postnatal age) to UDP-glucuronosyltransferase 1A6 (UGT1A6) and sulfotransferase 1A1 (SULT1A1) activity. 2. To evaluate the relationship of glomerular filtration rate to the elimination clearances of acetaminophen (APAP), APAP-glucuronide and APAP-sulphate, at different developmental stage (as measured by gestational and postnatal age) of the preterm neonate. 3. To evaluate the relationship of UGT1A6 genotype to UGT1A6 phenotype, and the relationship of SULT1A1 genotype to SULT1A1 phenotype. 4. To evaluate the relationship of developmental stage (defined by both gestational and postnatal age) to APAP-associated toxicities (as measured by the formation of APAP-CYS and other novel APAP toxicity- associated biomarkers) and the combined relationship to cumulative APAP dose.